1. Field of the Invention
This invention relates to a beam position sensor for a light beam scanner, and more particularly, to such a sensor which is adapted to be used with a light beam scanner in a laser printer.
2. Description of the Prior Art
Laser printers are used in photography and in the graphic arts for printing on a receiving medium such as film. When such printers are used for color imaging, they generally include a separate channel for each of the primary colors. In U.S. Pat. No. 4,728,965, for example, there is disclosed a laser printer which includes three optical channels, and each channel includes a gas laser which projects a beam of intense coherent light at a predetermined wavelength. The intensity of the light beam in each channel is modulated by an acoustooptic modulator in accordance with an electrical signal representing image information for one of the primary colors. The three beams of light are combined by a beam combiner, and the combined light beam is scanned onto a receiving medium by a rotating polygon.
In recent years, there have been attempts to use diode lasers instead of gas lasers in laser printers. The use of diode lasers reduces the cost of the printer and premits a drastic reduction in the size and complexity of the printer. Diode lasers can be modulated directly at frequencies as high as several hundred MHz by simply modulating the drive current, and thus, no external modulators are required. Further, the low intrinsic noise of diode lasers makes it possible to eliminate the high-bandwidth servo controls used for noise cancellation in gas laser printers. There are, however, problems which have to be solved in using diode lasers in printers for the graphics arts. One of the problems is that available gallium aluminum arsenide diode lasers emit in the infrared. As a result, a special recording material which is sensitive to the infrared must be used, and the diode lasers in a multilaser system must be carefully selected to obtain the widest possible spectral separation between the lasers.
It is common in laser printers which use a beam deflector, such as a polygon or a hologon, to use a line-start system which is adapted to accurately position the start of each raster line at the image plane. If a line-start system is not used, variations in the deflector rotational speed, or facet-to-facet variations, can cause uneveness in the cross-scan lines. Many line-start system use a detector which monitors the position of the scanned beam in order to start the printing of a raster line at the proper time. There are drawbacks, however, in systems which track the scanned beam, including the problem of detecting the scanned beam when it is going through the edge of the polygon facet. Any defects in the facet, such as an imperfect polygon corner, can affect the line start reading and hence the performance of the scanner. In order to overcome the problems in tracking the scanned beam in a line-start system, some systems provide a separate beam for the line-start system, as shown, for example, in U.S. Pat. No. 4,243,294.
U.S. Pat. No. 4,243,294, discloses apparatus for generating a synchronizing signal for a beam scanner which provides two angularly displaced light beams incident on the same reflecting mirror surface. A first beam is used for forming a scan line and a second beam is used for generating a synchronizing signal. The two beams are oriented such that when the first beam is reflected to the vicinity of the start point of a scan line, the second beam is reflected by the same mirror surface to a photodetector which generates a synchronizing signal. A problem with this apparatus is that both of the beams are generated in the same plane, and as a result, a relatively wide area is needed for the two light sources and the beam detector; this makes the apparatus too bulky for use in compact diode laser printers. A further problem is that interference between the two systems can occur when both beams are operating in the same plane.